1. Field of the Invention
This invention relates to an electrophotographic printing device with a toner developer unit, an exposure device, a developer drum, a photo-conductor, a transfer unit and a grounded charging device, wherein the substrate to be imprinted is moved, lying on a transport device, beyond the transfer zone of the transfer unit and the toner image of the transfer unit is transferred to the substrate.
2. Discussion of Related Art
A printing device is known from German Patent Reference DE 198 49 500 A1. The developer unit operates with a toner and is assigned to a photo-conductor drum. The surface of the photo-conductor drum is activated by an exposure device so that an application of toner to it becomes possible. The photo-conductor drum is connected via a contact line with a transfer roller. The transfer roller rolls off on the surface of the substrate to be imprinted and is transferred to the top of the substrate facing the transfer unit, using an electrostatic charge of the substrate.
Two transfer operations of the toner image occur in this printing device. The first transfer operation is created during the transfer from the photo-conductor drum to the transfer roller, and the second transfer operation during the transfer of the toner to the substrate. There is no complete transfer of the toner during each of the transfer operations. The achievement of as high as possible a rate of transfer should be attempted so that clear printed images with sharp contours are created. Thus the even and sufficient formation of the charge image in the area of the surface of the substrate, such as the charge transfer from the charging device to the substrate, is important.
Insufficient charging occurs in particular with thick substrates of a material with poor electrical conducting properties.